Re: mutexes, locks and so on...

[Matt Thomas <matt%3am-software.com@localhost>]

On Nov 16, 2010, at 7:32 PM, Johnny Billquist wrote:

> On 2010-11-17 04:25, matthew green wrote:
>>> The (my) problem is that rwlocks must use CAS as well, and I'm starting
>>> to think that I have to use CAS for the mutex code as well, as I can't
>>> seem to get mutexs work reliably without using the default
>>> implementation. The mutexes are used and abused in ways that seems to
>>> make a lot of implicit assumptions on the mutexes which go beyond what I
>>> might expect. Still working on it, though.
>> 
>> can you expand upon the problems?  i'm curious if they match what
>> we found with sparc long ago (but my memory is not really working
>> well for that right now... :)
> 
> I did sum up what I've found out so far, but as the code is still not working 
> reliably, my observations should be taken with a grain of salt.
> 
> But first of all, any mutex seems to be both taken and release through both 
> mtex_enter and mutex_spin_enter, and it can be done assymetrically. So a 
> mutex might be taken with mutex_spin_enter, and then released with mutex_exit.

Not true.  Spinlocks must enter through mutex_spin_enter and adaptive mutexes
enter through mutex_enter.  The corresponding is true for exiting as well.
The only reason mutex_vector_{enter,exit} is called to cause a panic.

> Also, the taking of spin mutexes can be done in mutex_vector_enter, and that 
> will use a definition for how a spin mutex is taken based on a macro that is 
> defined within kern_mutex.c, so spin mutexes can be dangerous for your sanity.
> 
> I have gotten the code to work, but a few minutes after boot, the system gets 
> stuck on a spin mutex already held. The code I replaced don't even check for 
> that condition, so I'm not sure what is going on here, but it might be that 
> there is some semi-obscure bug that was not causing any real problems 
> previously.

The VAX spin mutex code doesn't really need to change.  It's the adaptive mutex 
that's
a challenge for MP.  The RAS CAS used on UP is likely going to be faster than 
any 
alternative that you may come up with.